((x-3)*1.8+10)-((x-3)*2+8)
{10+1.8(x-3)}-{8+2(x-3)} ={10+1.8x-5.4}-{8+2x-6}=10+1.8x-5.4-8-2x+6=-0.8x+2.6
10+1.8*(X-3)=8+2*(X-3)可得余大X=130.2*(X-3)-2……(X>数山=13)薯毁中2-0.2*(X-3)……(X<=13)